Re: Protect syscache from bloating with negative cache entries

me> First off, I thought that I managed to eliminate the degradation
me> observed on the previous versions, but significant degradation (1.1%
me> slower) is still seen in on case.

While trying benchmarking with many patterns, I noticed that it slows
down catcache search significantly to call CatCacheCleanupOldEntries()
even if the function does almost nothing. Oddly enough the
degradation gets larger if I removed the counter-updating code from
SearchCatCacheInternal. It seems that RehashCatCache is called far
frequently than I thought and CatCacheCleanupOldEntries was suffering
the branch penalty.

The degradation vanished by a likely() attached to the condition. On
the contrary patched version is constantly slightly faster than
master.

For now, I measured the patch with three access patterns as the
catcachebench was designed.

master patched-off patched-on(300s)
test 1 3898.18ms 3896.11ms (-0.1%) 3889.44ms (- 0.2%)
test 2 8013.37ms 8098.51ms (+1.1%) 8640.63ms (+ 7.8%)
test 3 6146.95ms 6147.91ms (+0.0%) 15466 ms (+152 %)

master : This patch is not applied.
patched-off: This patch is applied and catalog_cache_prune_min_age = -1
patched-on : This patch is applied and catalog_cache_prune_min_age = 0

test 1: Creates many negative entries in STATRELATTINH
(expiration doesn't happen)
test 2: Repeat fetch several negative entries for many times.
test 3: test 1 with expiration happens.

The result looks far better, but the test 2 still shows a small
degradation... I'll continue investigating it..

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center